1. Field of the Invention
The present invention relates to an ink composition suitably used for inkjet recording and an inkjet recording method.
2. Description of the Related Art
With regard to an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photosensitive drum by electrically charging and exposing is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated.
On the other hand, with regard to the inkjet system, the equipment is inexpensive and, since an image is formed directly on a recording medium by discharging an ink composition only on a required image area, the ink composition can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.
In recent years, a solvent-free type inkjet recording method in which drawing by inkjet is carried out using an ink composition that is curable by irradiation with radiation such as UV rays (radiation curing type ink composition) and the ink is then cured by irradiation with radiation such as UV rays has been attracting attention.
In general, compared with an aqueous ink containing water as a diluent or a solvent-based ink containing an organic solvent as a diluent, an inkjet recording method employing a radiation curing type ink makes it possible to draw on a non-absorbing recording medium such as a glass, metal, or plastic recording medium, and has merits such as a wide recording medium application range, excellent scratch resistance and solvent resistance of a drawn image, high sensitivity, excellent productivity and, because it does not contain a volatile solvent, a low burden on the environment.
The curing mechanism for radiation curing type inks can be roughly divided into the radical polymerization type and the cationic polymerization type, and since the radical polymerization type has merits such as excellent storage stability and low cost compared with the cationic polymerization type, it is widely used in the market.
Radical polymerization type ink compositions are described in, e.g., JP-PCT-2001-525479 (JP-PCT denotes a published Japanese translation of a PCT application), JP-PCT-2004-514014, and JP-A-2006-299117 (JP-A denotes a Japanese unexamined patent application publication). Since a printed material recorded by radiation-curing type inkjet as above is often used in industrial applications such as outdoor advertisements, high lightfastness and saturation are required.
On the other hand, with regard to a radiation-curing type ink for inkjet recording, an ink set is formed using yellow, magenta, cyan, and black as four basic colors, and since the yellow color has poor lightfastness compared with the other colors, development of a yellow ink having both high saturation and lightfastness is an important task.
In order to improve the problem of the yellow ink, JP-A-2004-2528 and JP-A-2006-348206 below have made proposals.
JP-A-2004-2528 discloses a UV-curing type inkjet recording ink composition comprising C.I. Pigment Yellow 180 as a yellow pigment, a polymeric dispersing agent having a basic adsorbing group, a photopolymerizable compound, and a photopolymerization initiator.
JP-A-2006-348206 discloses an actinic radiation-curing type inkjet ink comprising an isoindoline pigment, an isoindoline derivative containing a sulfonic acid or a salt thereof, a polymeric dispersing agent, and a monomer.